I signed up to buy Hexy a few days ago, and my R-PI is somewhere in the East Midlands according to DHL.
In terms of sharing batteries, I suspect it is not a good idea for the R-PI to use the 6 volt battery that powers Hexy (they will be providing 2 battery packs, a 4 AA pack if you use non-rechargeable batteries that give off 1.5 volts each, and a 5 AA pack if you use rechargeable that give off 1.2 volts each to get to 6 volts). Servos require a lot of power, and it is common that you can't control the Arduino and servos off of the same 5v supply. I experienced this myself, when I put an LCD screen on my Arduino, I could no longer run the single servo I had in the circuit.
In theory, you could get a 12v battery, and have 2 outputs, one at 6v for the Hexy, and one at 5v for the R-PI. I see some that have 12/9v output along with a 5v USB port. Or just have 2 batteries, 1 for each processor.
According to this: http://arcbotics.com/products/servotor32/
the Servotor32 platform that is at the heart of Hexy can be controllable via USB, so it should controllable from the R-PI. Given all of the source code will be available, it should be mod-able.